This invention relates to a siliceous perlite product having controlled particle size distribution, which is useful in filter and filler applications.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification. The disclosure of the publications, patents, and published patent specifications referred in this application are hereby incorporated by reference into the present disclosure.
Many methods for the separation of particles from fluids employ cellular or porous siliceous media, such as diatomite or perlite, as filter aids (Bear, 1988; Cain, 1984; Carman, 1937; Heertjes, 1949, 1966; Ruth, 1946; Sperry, 1916; Tiller, 1953, 1962, 1964).
Perlite products have been prepared by milling, screening, and thermal expansion. Depending on the quality of the perlite ore and the method of processing, expanded perlite products have been used as filter aids, lightweight insulating materials, filler materials, horticultural and hydroponic media, and chemical carriers; expanded perlite has been used in filtration applications since about the late 1940""s (Breese and Barker, 1994). Expanded perlite is also used as an absorbent for treating oil spills (e.g., Stowe, 1991).
Conventional processing of perlite consists of comminution of the ore (crushing and grinding), screening, thermal expansion, milling, and air size separation of the expanded material to meet the specification of the finished product. For example, perlite ore is crushed, ground, and separated to a predetermined particle size range (e.g., passing 30 mesh), then the separated material is heated in air at a temperature of 870-1100xc2x0 C. in an expansion furnace (cf. Neuschotz, 1947; Zoradi, 1952), where the simultaneous softening of the glass and vaporization of contained water leads to rapid expansion of glass particles to form a frothy glass material with a bulk volume up to 20 times that of the unexpanded ore. The expanded perlite is then air separated to meet the size specification of the final product. The expanded perlite product may further be milled and separated for use as filter aid or filler material (Breese and Barker, 1994). Some degree of separation after expansion is common, for example, using cyclones, which are simple conical devices that separate particles according to their aerodynamic mass by suspending them in a stream of air. Stein (1955) states that two or more cyclones in series are sometimes used, the first cyclone having lower efficiency than the following ones, so as to air-separate the product into several size fractions.
Expanded perlite products have found widespread utility in filtration applications. The principles of filtration using porous media have been developed over many years (Carman, 1937; Heertjes, 1949, 1966; Ruth, 1946; Sperry, 1916; Tiller, 1953, 1962, 1964), and have been recently reviewed in detail from both practical perspectives (Cain, 1984; Kiefer,1991) as well as from their underlying theoretical principles (Bear, 1988; Norden, 1994).
Perlite products are applied to a septum to improve clarity and increase flow rate in filtration processes, in a step sometimes referred to as xe2x80x9cprecoating.xe2x80x9d Perlite products are also added directly to a fluid as it is being filtered to reduce the loading of undesirable particulate at the septum while maintaining a designed liquid flow rate, in a step often referred to as xe2x80x9cbody feedingxe2x80x9d. Depending on particular separation involved, perlite products may be used in precoating, body feeding, or both. Perlite products, especially those which are surface treated, can enhance clarification or purification of a fluid (Ostreicher, 1986). Expanded perlite products are often used as insulating fillers, resin fillers, and in the manufacture of textured coatings (Breese and Barker, 1994).
Particle size has strong effect on both filter aid and filler applications. The selection of the proper grade of filter aid depends on the size of the suspended particles that are to be removed. It is axiomatic in the use of filter aids that as the filter aid particle size and the liquid flow rate increase, the ability of the filter aid to remove small particles of suspended matter decreases (Kadey, 1983). Conversely, as filter aid particle size and liquid flow rate decrease, the ability of the filter aid to remove small particles of suspended matter increases. The performance of fillers is also closely related to the particle size. Particle size effects result from the average size, the top size, and the size distribution. For example, in a paint system, the average size of a filler may affect viscosity and binder demand, the size distribution may affect packing and resultant density and film integrity, and the top size may affect paint gloss/sheen and smoothness, or cause fracture failures or cracking (Trivedi and Hagemeyer, 1994).
The as-expanded perlite normally contains a significant amount of light-weight material called floaters, which are expanded perlite particles that often contain entrapped air. As the name suggests, floaters quickly float to the surface of liquid rather than remain buoyantly suspended in it. The floater content has a negative effect on filter aid and filler applications because they often float away from their intended functional location, for instance, either away from the filter septum or away from a desired location within a filler system. Therefore, floaters should be minimized during the processing of perlite for many applications.
Color is also important for a filler in any application, especially where color of the end product is important. Whiter filler products with high blue light brightness normally have greater utility, as they can be used in all colored and white products and, relative to non-white fillers, reduce the demand for expensive white pigments, such as titanium dioxide. For these reasons, perlite products with controlled particle size distribution, low floater content, and high blue light brightness are often desirable.
The particle size distribution, floater content and blue light brightness of commercially available expanded perlite products, measured using the standard methods described below, are shown in Table I. The lowest median particle size (d50) for the product having a ratio of the standard deviation of particle size distribution to the median particle size (sd/d50) less than 0.63 is 53 microns (sometimes abbreviated xe2x80x9cxcexcxe2x80x9d; i.e., xcexcm, or micrometer). The lowest floater content for the product having a ratio of the standard deviation of particle size distribution to the median particle size (sd/d50) less than 0.63 is 10 percent by volume.
Fine expanded perlite products typically have a median particle size of 40 xcexcm or less. The production of fine, expanded perlite products of a controlled particle size distribution having low wet density has been difficult due to the characteristics of expanded perlite and the limitations of existing commercial mineral milling equipment. For example, when unexpanded perlite ore is crushed and screened to a size finer than approximately 54 micrometers, and is then expanded, it forms virtually 100% microspheres (sometimes referred to as microbubbles), a form of floaters, which are hereinafter referred to as expanded perlite microspheres. As a result, fine, low wet density perlite products containing a low percentage of floaters are not generally produced by expanding finely milled ore. Conventional perlite products that are fine and relatively lower in floater content are produced by one of two methods: (i) expanding perlite to a coarser size than desired and milling the expanded product to the desired size; or (ii) recovering a small percentage of the by-product fines produced during the milling of coarser products in a fines recovery circuit.
When a coarse expanded perlite product is milled by conventional means, the wet density of the product increases substantially. In addition, existing commercial milling and classifying equipment is designed for much denser materials and does not provide good control of the particle size of milled light density materials, such as expanded perlite. Fine expanded perlite with typically somewhat lighter wet density can be produced by recovering by-product fines from a conventional perlite milling operation, but the quality control of these products is difficult, yields are low, and perlite products with controlled particle size distribution are not generally produced.
There is a need for improved perlite products with controlled particle size distribution.
An expanded perlite product having a controlled particle size distribution is provided, wherein the ratio of the standard deviation of particle size distribution to the median particle size is less than 0.63; and wherein the median particle size is less than 50 microns.
In the expanded perlite product, for example, the ratio of the standard deviation of particle size distribution to the median particle size is less than 0.60; less than 0.58; or less than 0.55. The expanded perlite product has, for example, a wet density less than 50 pounds per cubic foot; less than 40 pounds per cubic foot; less than 35 pounds per cubic foot; less than 30 pounds per cubic foot; less than 25 pounds per cubic foot; or less than 20 pounds per cubic foot.
The expanded perlite product has, for example, a floater content of less than 10 percent by volume; a floater content of less than 5 percent by volume; a floater content of less than 2.5 percent by volume; or a floater content of less than 2 percent by volume. The expanded perlite product has, for example, a blue light brightness greater than 80; greater than 82; greater than 83; or greater than 85.
The expanded perlite product has, for example, a Hegman fineness greater than 1.0; greater than 2.0; greater than 3.0; greater than 4.0; greater than 5.0; or greater than 6.0.
Processes for the preparation of an expanded perlite also are provided, for example, comprising using air classification equipment to effect both milling and air classification, thereby to obtain the expanded perlite product. The expanded perlite product also can be obtained, for example, by centrifugal sieving.
Further provided are filters, insulating materials, fillers, horticultural media, hydroponic media, and chemical carriers comprising the expanded perlite product. Also provided are methods of separating components from a solution, comprising filtering a solution comprising the components through a filter comprising the expanded perlite products.